On the Local Charge Inhomogeneity and Lithium Distribution in the Superionic Argyrodites Li6PS5X (X = Cl, Br, I)

Nuclear magnetic resonance offers a wide range of tools to analyse ionic jump processes in crystalline and amorphous solids. Both high-resolution and time-domain 1 , 2 H , 6 , 7 Li , 19 F , 23 Na NMR helps throw light on the origins of rapid self-diffusion in materials being relevant for energy storage. It is well accepted that Li + ions are subjected to extremely slow exchange processes in compounds with strong site preferences. The loss of this site preference may lead to rapid cation diffusion, as is also well known for glassy materials. Further examples that benefit from this effect include, e.g. cation-mixed, high-entropy fluorides ( Ba, Ca) F 2 , Li-bearing garnets ( Li 7 La 3 Zr 2 O 12 ) and thiophosphates such as LiTi 2 ( PS 4 ) 3 . In non-equilibrium phases site disorder, polyhedra distortions, strain and the various types of defects will affect both the activation energy and the corresponding attempt frequencies. Whereas in ( Me, Ca ) F 2 ( Me = Ba , Pb ) cation mixing influences F anion dynamics, in Li 6 PS 5 X ( X = Br , Cl , I ) the potential landscape can be manipulated by anion site disorder. On the other hand, in the mixed conductor Li 4 + x Ti 5 O 12 cation-cation repulsions immediately lead to a boost in Li + diffusivity at the early stages of chemical lithiation. Finally, rapid diffusion is also expected for materials that are able to guide the ions along (macroscopic) pathways with confined (or low-dimensional) dimensions, as is the case in layer-structured RbSn 2 F 5 or MeSnF 4 . Diffusion on fractal systems complements this type of diffusion. This article is part of the Theo Murphy meeting issue ‘Understanding fast-ion conduction in solid electrolytes’.

Section: (C) Cation Disorder Influencing Anion Dynamics and Vice Versasupporting

confidence: 62%

Section: (C) Cation Disorder Influencing Anion Dynamics and Vice Versamentioning

confidence: 99%

Fuzzy logic: about the origins of fast ion dynamics in crystalline solids

Gombotz

Hogrefe

Zettl

et al. 2021

“…Finally, we point out that the disordered mixing of anions in solid solutions and alloys on an otherwise static lattice can also create an energy landscape that is intrinsically structurally frustrated and unable to accommodate cation coordination preferences. This phenomenon has been extensively cited in the context of the argyrodites [37,61,[71][72][73][74][75][76][77][78][79][80][81][82]. In these systems, structural disorder between the sulfur and halide anions can lead to disruption of the local cation coordination environment and hence increased diffusivity.…”

Section: (B) Site Distortion and Anion Packingmentioning

confidence: 98%

Paradigms of frustration in superionic solid electrolytes

Wood

Varley

Kweon

et al. 2021

Superionic solid electrolytes have widespread use in energy devices, but the fundamental motivations for fast ion conduction are often elusive. In this Perspective, we draw upon atomistic simulations of a wide range of superionic conductors to illustrate some ways frustration can lower diffusion cation barriers in solids. Based on our studies of halides, oxides, sulfides and hydroborates and a survey of published reports, we classify three types of frustration that create competition between different local atomic preferences, thereby flattening the diffusive energy landscape. These include chemical frustration, which derives from competing factors in the anion–cation interaction; structural frustration, which arises from lattice arrangements that induce site distortion or prevent cation ordering; and dynamical frustration, which is associated with temporary fluctuations in the energy landscape due to anion reorientation or cation reconfiguration. For each class of frustration, we provide detailed simulation analyses of various materials to show how ion mobility is facilitated, resulting in stabilizing factors that are both entropic and enthalpic in origin. We propose the use of these categories as a general construct for classifying frustration in superionic conductors and discuss implications for future development of suitable descriptors and improvement strategies. This article is part of the Theo Murphy meeting issue ‘Understanding fast-ion conduction in solid electrolytes’.

“…Effective Li + transport, however, can only be achieved via intercage jumps. These have been reported to be the bottleneck for long-range diffusion [9] and seem to be facilitated by T2 and potentially also T4 sites [8,10].…”

Section: Introductionmentioning

confidence: 99%

Influence of Br ⁻ /S ²⁻ site-exchange on Li diffusion mechanism in Li ₆ PS ₅ Br: a computational study

Sadowski

Albe

2021

We investigate how low degrees of Br − / S 2 − site-exchange influence the Li + diffusion in the argyrodite-type solid electrolyte Li 6 PS 5 Br by ab initio molecular dynamics simulations. Based on the atomic trajectories of the defect-free material, a new mechanism for the internal Li + reorganization within the Li + cages around the 4 d sites is identified. This reorganization mechanism is highly concerted and cannot be described by just one rotation axis. Simulations with Br − / S 2 − defects reveal that Li + interstitials ( L ii . ) are the dominant mobile charge carriers and originate from Frenkel pairs. These are formed because B rS . defects on the 4 d sites donate one or even two L ii . to the neighbouring cages. The L ii . then carry out intercage jumps via interstitial and interstitialcy mechanisms. With that, one single B rS . defect enables Li + diffusion over an extended spatial area explaining why low degrees of site-exchange are sufficient to trigger superionic conduction. The vacant sites of the Frenkel pairs, namely V Li ′ , are mostly immobile and bound to the B rS . defect. Because S Br ′ defects on 4 a sites act as sinks for L ii . they seem to be beneficial only for the local Li + transport. In their vicinity T4 tetrahedral sites start to get occupied. Because the Li + transport was found to be rather confined if S Br ′ and B rS . defects are direct neighbours, their relative arrangement seems to be crucial for effective long-range transport. This article is part of the Theo Murphy meeting issue ‘Understanding fast-ion conduction in solid electrolytes’.